The process of creating a vapor from a liquid usually involves heating the liquid to a sufficiently high temperature to form vapor. In semiconductor device fabrication, the drying of wafer using isopropyl alcohol (IPA) vapor is an important and well-known water processing step (see U.S. Pat. Nos. 5,315,766, 5,369,891, 5,571,337, 5,671,554, 5,855,077 and 6,029,371). In this process, the vaporized IPA is allowed to condense on the wafer surface to wash away molecular and particulate contaminant on the surface. The result is a clean wafer surface suitable for the subsequent wafer processing steps in semiconductor integrated-circuit device manufacturing.
FIG. 1 shows a traditional approach to vapor generation by means of a boiler. The boiler, shown generally located at 100, includes a vessel filled with liquid IPA to level 120, an immersion heater 110 to supply the needed heating energy to vaporize the liquid, and an outlet 130 for the vapor to flow out and exit the boiler. During the vaporization process, a great deal of heat is transferred from the heater to the liquid to cause bubbles 115 to form in the liquid as depicted. When a bubble 125 rises to the liquid surface 120 the bubble would burst to form a cloud of small droplets. The droplets then evaporate leaving behind small non-volatile residue particles 130 suspended in the vapor. These residue contaminant particles would then be carried by the vapor flowing out of the boiler through outlet 135 to contaminate wafers located downstream, leading to the loss of product yield in the subsequent wafer processing steps.